PT Output

[TIMBO]

Hi guys, Thought I'd better check as it will produce a high output.
The Hammond 369kx
http://www.hammondmfg.com/pdf/EDB369KX.pdf
Has approx 450v across the winding and I'm looking for approx. 620v after rectification. Using a standard bridge rectifier( no CT connected) to achieve this ok, I would have to cover the voltage and current of course.
Anything I need to do safety/noise wise. Thanks

[HotBluePlates]

The 50vac bias tap will be no use to you, and the high voltage winding will now only supply 150mA. Other than that, it seems fine!

[mresistor]

HPB - wouldn't the current rating be dependent on the load, whether it is capacitive or inductive input?   Reason I ask is because I was looking at this  http://www.hammondmfg.com/pdf/5c007.pdf

[Lauri]

The 50vac bias tap will be no use to you, and the high voltage winding will now only supply 150mA. Other than that, it seems fine!

Why would 300mA 450V center tapped winding only supply 150mA? That's a 188VA transformer. If the 450V winding were only 150mA then there would be almost 70VA missing.

[PRR]

PTs can be rated for AC VA or for DC output with specified (or implied) rectifier.

AC VA has to be de-rated to get DC power, especially with cap-input rectifier which pulls huge current spikes. The usual factor is 1.6 to 1.8. (This works against the 1.414 voltage factor.) There's notes in the cheat-sheet link above.

The 225-0-50-225 configuration "implies" a 2-diode rectifier. Each half winding is working only half the time.

In general, Hammond specifies this type of winding for _DC_ current (not VA).

So a 4-diode bridge using 450VAC "should" give double voltage at half current; 150mA.

The correction is not exactly half. The primary heat is the same either way. 200mA may not heat or sag bad.

Hammonds are quite conservative. And large audio amps are not full-power all the time. 250mA max may be fine.

Choke-input may allow more current. However less voltage: 450VAC to a choke-input filter gives 400V DC. I suspect this part is specified for cap-input, because who needs that much 200VDC?
______________

> looking for approx. 620v
> That's a 188VA

600+V DC sure smells like a 100 Watt output. (However ~60W with 6.6K loading as in Ampeg VT40.)

Tube power amps rarely beat 50% efficiency. So we want 200W of DC.

Plus rectifier conversion factors.

Plus heaters.

I suspect 188VA is not ample for a 100W amplifier. It may do for a 60W job.

[Paul1453]

Timbo,

You didn't tell us what tubes you plan on hammering with +620VDC.   

I too, suspect you are going for a very high wattage output amp here.
To me, that's pretty much anything over 50 Watts.   

It's extremely nice to have a real Engineer like PRR to give us the low down on things like this.   

[TIMBO]

Thanks guys, Looking to make another Maxim clone and just sussing a PT for the job.
4x12AX7, 1xEF86, 2x6SK7 and 2x 6550.
I have mine bias to 60w.
Looking to match up with a Hammond 1650NA.
http://www.hammondmfg.com/pdf/EDB1650NA.pdf

[Lauri]

PRR, thank you for explaining this. So just to make sure I've understood everything correctly. When using a full wave bridge rectifier with cap input the ac current rating of the transformer secondary winding should be 1.6 to1.8 times higher than the dc current load. But it's better to make it 2 times higher to avoid heat and voltage sagging.

[PRR]

> a Hammond 1650NA

Rated 60W output. 4.3K. UL.

You sure do not want 600V on 6550 into 4.3K. Rated load for the high voltages is 5K. If you did load them in 4.3K, and jacked G2 to make it work, output is 116W, near twice what the OT says.

4.3K at 60W output is nearer 450V B+. I've interpolated below. Proper G2 is near 250V, which is awkward. If you let G2 run near raw B+, bias has to be much larger to keep a reasonable idle heat.

You are essentially spot-on the 6550 70W UL specs. UL for guitar brings up various opinions. But it is a smooth well-behaved mode. IAC, the UL bias gives a cross-check on my 440V G2 bias estimate.

[TIMBO]

Thanks PRR, Again you have diverted a huge problem.
There is a lot more to it than I know.
Because of the big lineup of tubes I need at least 6A for the heaters.
And using the transformers on my build (maxim clone) I was using it as a guide.
The PT I am able to get locally and there is a limited choice.
It's hard to get one that has enough heater current as well as supply current.
This is probably my next best choice http://www.hammondmfg.com/pdf/EDB372HX.pdf
Has the heater current, supply current but a bit low on the B+.
Whatdayathinx?

[PRR]

For 60W out with a 4.3K load, I would be looking at Bassman PTs. It's pretty close.

Also Marshall "50W" PTs, because they have some heater power.

Do you really need 6A? That's two 6550 and like nine 12A_7.

[TIMBO]

Thanks PRR, 4x12ax7, 1xEF86, 2x6SK7 and 2x6550.
Just over 5A's do you think we can get away with it.